U.S. researchers said survey results show last year's flu vaccine fell far short, compared to past years, in keeping people from getting the virus.

Harris Interactive said the online survey of 2,529 U.S. adults confirmed the findings by the Centers for Disease Control and Prevention that the vaccine was less effective than the flu vaccines used in previous winters.

In three previous winters, people receiving flu vaccines were 33 percent, 43 percent and 24 percent less likely to get the flu than people who did not get flu shots. This year, people who received flu shots were just as likely to get the flu as those who did not get flu shots, Harris Interactive said in a release.

The CDC report published last month said the flu viruses most prevalent this past winter turned out to be different than the viruses used in the vaccine.

Harris said the survey probably understated the effectiveness of flu shots because people who are more likely to get the flu may also be more likely to get their flu shots. Harris also said it is difficult for patients and physicians to make a certain diagnosis of flu. (c) UPI